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projects > ecosystem history: florida bay and the southwest coast > abstract


Molluscan Fauna as Indicators of Change in Florida Bay and Biscayne Bay

G. Lynn Brewster-Wingard


Analysis of seven cores from Florida Bay and Biscayne Bay has indicated significant changes in the molluscan fauna during the latter half of the 20th century. Brachidontes exustus comprises >80% of the molluscan fauna in the upper portions of six cores. In four of the cores, this increased dominance of Brachidontes is unprecedented. Brachidontes exustus, a nearly ubiquitous mussel at current sites in eastern and central Florida Bay, can tolerate diminished water quality and a wide range of salinities. Five cores also show dramatic decreases in molluscan abundance and diversity during the last forty years. These findings indicate a system under stress.

Molluscs as a group possess many characteristics that make them ideal as proxies for the general health of estuarine ecosystems. They occupy a number of trophic levels as primary, secondary, and tertiary consumers and they are an integral part of an estuarine food web. A number of feeding strategies are utilized by molluscs, including filter feeding, grazing, scavenging, and carnivory. Throughout their lifetime, some species of molluscs may pass from the planktonic to the benthic realm, and they move from micro-, to meio-, to macro-fauna as they mature. A decrease in molluscan diversity and abundance may be an indication of a disrupted system.

Molluscs generally live in equilibrium with the surrounding water, but have the ability to tolerate water outside their normal range for brief periods by halting their exchange with the water column. Because they typically exist at equilibrium with seawater, they record the composition of seawater at the time the shell is deposited. Molluscs generally are tolerant of short-term perturbations in their environment, but they will respond to seasonal and annual changes in salinity. Thus, changes in salinity and other measures of water quality indicated by molluscan shell chemistry or molluscan assemblages, represent significant changes to the environment.

Key molluscan indicator species and whole assemblages therefore provide vital historical data on the following:

  1. salinity and proximity to freshwater outflow;
  2. water quality;
  3. substrate, including the density and distribution of seagrass and macrobenthic algae;
  4. estimates of overall diversity and health of the ecosystem;
  5. sea level and/or proximity to shoreline.

In addition, the shells of individual molluscs serve as archives of changing water chemistry. Experimental work has begun this year on extracting monthly and seasonal salinity and other water chemistry data from mollusc shells, with emphasis on the relatively thick-shelled species, Chione cancellata.

Analyses of molluscan assemblages to date have contributed the following historical information.

  • The environments at the mouth of Taylor Creek in Florida Bay (core T24), and in Manatee Bay (core MB1) show distinct shifts from oligohaline/mesohaline assemblages with a strong freshwater influence, to polyhaline assemblages with very little freshwater influence. Yet the changes have not been enough to displace Anomalocardia auberiana, a species typical of the northern transition zone in Florida Bay; this species is present throughout both cores.
  • Pass Key (core PK37), southeast of Little Madeira Bay, shows significant fluctuations in species indicative of the northern transition zone, probably indicating fluctuations in salinity at the site.
  • Between 1910 and 1930, distinctive changes occur in the molluscan assemblage at Russell Bank (core RB19B). Anomalocardia auberiana, consistently present in low percentages prior to 1920, is not present in the upper portion of the core, which may indicate a decreasing influence from Taylor slough and the northern transitional area.
  • The dominance of Brachidontes exustus, beginning around 1980 at Russell and Bob Allen, around 1950 in Whipray (core 25B), and in the upper portion of the cores from Pass Key, Taylor, and Manatee Bay (core MB1), could be an indicator of diminished water quality, increased fluctuations in salinity, or any combination of factors stressing the molluscan fauna of the region. The only core examined that does not show an increase in Brachidontes dominance in the upper portion of the core is FB1 from Featherbed Bank in Biscayne Bay. Brachidontes is very rare at this site, which is dominated by more euhaline faunas.
  • The four cores from central and eastern Florida Bay, and from Featherbed Bank in Biscayne Bay all show decreases in measures of molluscan faunal diversity and absolute abundance that may have begun in the 1960's or earlier, and reached a low in the early 1970's. Interestingly the transition zone cores from the mouth of Taylor Creek and in Manatee Bay do not show this decline.

Experimental work with living molluscs will provide additional data to interpret changes in the downcore assemblages, and is providing the framework for future work analyzing the change in shell chemistry within an individual mollusc shell. If successful, the planned analyses of mollusc shells from well-dated cores will allow determination of monthly, seasonal and annual changes in water chemistry. These data will provide target data for the restoration efforts to restore historical seasonal flow into Florida Bay and Biscayne Bay.

(This abstract was taken from "Programs and Abstracts - 2001 Florida Bay Science Conference". (PDF, 6.8 MB))

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Last updated: 11 October, 2002 @ 09:30 PM (KP)